The use of insertion devices such as undulators and wigglers with charged particle beams for the generation of electromagnetic radiation, particularly x-rays, has become increasingly common in recent years. A prior art insertion device typically consists of two linear arrays of magnets located on opposite sides of a portion of a beam of relativistic charged particles. As the particles pass between the magnets, the particles are subjected to an alternating magnetic field which causes the particles to be accelerated in directions transverse to the beam direction. This alternating acceleration causes the particles to emit electromagnetic radiation. The shape of the energy spectrum of the emitted radiation depends on the number and amplitude of oscillations to which the beam is subjected and the detailed arrangement of the magnets in the arrays. The amplitude of the oscillations depends on the magnetic field strength in the region between the arrays of magnets.
It is often advantageous to provide a source of x-rays whose polarization and characteristic energy may be varied. X-ray sources are useful in both spectroscopic and fixed energy applications. In imaging applications, it is often advantageous to construct an image by subtracting two component images that were generated by illuminating the specimen with radiation having different polarizations. Similarly, measurements of the magnetic dichroism of materials such as magnetic recording media require measurements of the response of the specimen to radiation having different polarizations. Usually, the differential measurements are made using radiation having either left or right handed circular polarization. To obtain the maximum contrast, the radiation source must provide radiation which is substantially of one polarization.
The optimum energy for the radiation source will, in general, depend on the experiment being performed. Hence, it is advantageous to provide a radiation source in which the energy of the source may be varied. In general, the x-ray energy is varied by varying the magnetic field strength in the insertion device or by varying the energy of the charged particles in the beam. In the prior art systems in which the magnetic field strength is varied, the field strength is adjusted by employing electromagnets and varying the current therein or by employing permanent magnets and varying the distance between the two rows of magnets. Permanent magnets have been found to be more attractive than electromagnets because they provide high field density without the need for cooling.
The need to vary the gap in permanent magnet systems leads to structural and mechanical problems. The new generations of x-ray sources may require insertion devices of 5 meters or longer with gaps less than 30 min. In addition to the problems of moving and aligning a device of this size which may weigh several tons, the positioning apparatus must withstand the force of attraction between the two rows of magnets. For example, an exemplary 4 meter insertion device with a minimum gap of 30 mm must resist forces in excess of 91 kN. The structural and mechanical problems inherent in providing a means for controlling the positioning and alignment of such a device will be apparent to those skilled in the mechanical arts.
Prior an systems for generating elliptically polarized x-rays have various limitations as to purity of polarization and as to flux. Quarter wave plate and related techniques are limited as to the range of energies at which they may be used. Bending magnet techniques, the most common in use, display sharply decreasing flux at higher rates of circular polarization. Variable gap insertion device techniques may suffer from certain mechanical and electron optical complications. Mechanical complications arise from the requirement that the gap variation must be done with great precision against very large forces. Electron optical effects include susceptibility to very large forces. Electron optical effects include susceptibility to horizontal beam steering errors and tune shifts due to changes of vertical electron beam focusing with gap.
Broadly, it is the object of the present invention to provide an improved insertion device.
It is a further object of the present invention to provide an insertion device that utilizes permanent magnets while avoiding the mechanical and structural problems inherent in controlling the gap between the two rows of magnets.
It is yet another object of the present invention to provide an insertion device which allows the energy and polarization of the generated radiation to be changed without changing the gap between the rows of magnets.
It is still a further object of the present invention to provide an insertion device that minimizes variations in the vertical focusing or horizontal steering to the particle beam when the magnetic field to which the particles are subjected is altered.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.